The present invention relates to an image forming apparatus which converts electric information to photo-information to record in a recording medium such as a photoconductor.
Printing apparatus in which electrophotographic technology is applied include laser printers in which a photosensitive drum is irradiated with a laser beam using a rotary scanning mirror (a polygon mirror), and LED printers wherein light emitting diodes (LEDs) corresponding to picture elements are arranged in the scanning direction of the recording medium and in a vertical direction to expose a photosensitive drum to the LED light,
FIG. 16 is a view showing a conventional LED printers, photosensitive drum (recording medium) 101 and which uniformly provides the surface of the photosensitive drum with charges. A LED head 102 converts input electric signals to optical ON/OFF signals to which the photosensitive drum 101 is exposed through a lens array 103 without reducing/magnifying. As a result, an electrostatic latent image corresponding to the electric signals input to the LED head 102 is formed on the surface of the photosensitive drum. The photosensitive drum surface where the electrostatic latent image is formed interacts with a developing apparatus 104 to which a bias voltage is applied,. and toner is transferred to the photosensitive drum surface to develop the electrostatic latent image. The photosensitive drum surface developed by the toner is rotated to reach a transcribing portion. On the other hand, a paper sheet 107 is fed from a sheet feeding portion 106 to the transcribing portion, and the toner on the photosensitive drum surface is pulled by a corona ion having a polarity reverse to that of the charge of the toner onto the photosensitive drum surface, and discharged from a transcription charger 105 to form a latent image on the paper sheet 107. However, the image on the printing paper sheet 107 in this state is not stable because it is electrostatically deposited. A stable image is obtained in such a manner that the toner permeates the paper sheet by heating and melting using a fixing apparatus 108.
FIG. 17 is a view showing an internal structure of an embodiment of the LED head 102. The embodiment of the LED head 102 is a head used for a printer having a resolution of 300 dots per inch (dpi), which can be obtained by arranging LEDs having four sides of 60 .mu.m at pitches of 85 .mu.m. It is necessary to arrange about 2400 LEDs at the pitches in order to obtain an image of the full length of the longer side of an A4 size paper sheet, having a resolution of 300 dpi. This necessity is conventionally satisfied in such a manner that 38 LED chips 110, each having 64 rectangular LEDs 109 with each LED having four sides of about 60 .mu.m and arranged at about 85 .mu.m pitches, are accurately arranged and fixed on a substrate 111. Each LED 109 of the LED chip 110 is connected to a driving circuit chip 113 via a bonding wire 112 and are independently controlled with an input signal input from an input connector 114.
Such a conventional constitution, however, has the following drawbacks.
1) Since an industrially manufacturable LED chip is limited in size for cost reasons, it is not possible to constitute a LED head having conventional sizes (e.g., identical with the shorter side of a A4 size paper sheet) using a single LED chip.
The LED chip is manufactured by repeating etching of a GaAs or GaAl wafer similar to the manufacture of an IC. According to industry and trade convention, a 10 in. ingot (an ingot is sliced to make wafers) is the maximum size of an ingot for manufacturing a wafer. When a LED chip is manufactured from a 10 in. wafer, the largest practical size of a LED chip is about 241 mm. When the LED chip is mass-produced, the largest practical size thereof is about 8 inches. Accordingly, the LED head 102 is conventionally manufactured with a plurality of small size LED chips 110 (about 38 chips). Such constitution, however, requires a high accuracy on the order of a few .mu.m in forming a row of the LED chips, which further requires a large size apparatus for manufacture. Even then the yield is unsatisfactory. For example, when a LED chip having a length of 241 mm (9.5 in.) and a resolution of 600 dpi is manufactured from a 10 in. wafer, it is difficult to obtain good quality from all of the 5700 elements manufactured on the LED chip.
2) Conventionally, the LED driving circuit chip 113 and the LED chip 110 are separate chips. Additionally, 2400 LEDs 109 are respectively wired to the driving circuits via bonding wires. Materials for manufacturing the driving circuit chip and LED chip are limited due to their characteristics, and there are no means available for forming both chips on the same substrate. Accordingly, the driving circuit chip and the LED chip are formed from a Si substrate and from a GaAs or GaAl substrate, respectively, and the wiring thereof is carried out by wire bonding. This practice lowers the reliability of the LED head increases cost because of the substantial time expended in manufacturing.